Recently, there has been an increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4-Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict fire and smoke emission and the like of electrochemical devices. In view of safety characteristics of electrochemical devices, overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin membranes commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage may cause a short circuit between an anode and a cathode.
In order to solve the above safety problems of electrochemical devices, an electrode having a porous coating layer formed by combining inorganic particles and binder on the surface of an active material layer of the electrode has been proposed. However, in this electrode, it is difficult to fully coat the porous coating layer on the surface of the active material layer, and there were still safety problems because the side of the electrode was exposed to the outside.